<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let num1 = +prompt('请输入第一个数')
    let num2 = +prompt('请输入第二个数')
    let num3 = +prompt('请输入第三个数')

    let operation = num1 >= num2 ? num1 :num2
    let max = operation >= num3 ? operation : num3
    alert(max)
  </script>
</body>
</html>